Thapsigargin induces an endothelium-dependent, intracellular calcium ion-dependent vasodilation in vitro

Endothelium-dependent noradrenergic hyperresponsiveness induced by thapsigargin in human saphenous veins: role of thromboxane and calcium

To further investigate the mechanisms which regulate sympathetic vascular tone, we studied the effects of the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin, on the vasoconstriction induced by transmural nerve stimulation and noradrenaline in superfused human saphenous vein rings. The contractions induced by both transmural nerve stimulation and noradrenaline were potentiated by thapsigargin in endothelium-intact, but not in endothelium-denuded vessels. This potentiation was unaffected by the non-selective endothelin ET(A/B) receptor antagonist, Ro 47-0203 (4-tert-Butyyl-N-[6-(2-hydroxy-ethoxy)-5-(2-methoxy-phenoxy)-2,2'-bipyrimidin-4yl]benzene sulfonamide), or by the nitric oxide (NO) synthase inhibitor, L-NNA (N(omega)-nitro-L-arginine), but was inhibited by the thromboxane A(2) receptor antagonist, Bay u3405 (3(R)-[[(4-flurophenyl) sulphonyl]amino-1,2,3,4-tetrahydro-9H-carbazole-9-propanoic acid]) or by the thromboxane A(2) synthase inhibitor, UK 38485 (3-(1H-imidazol-1-yl-methyl)-2-methyl-1H-indole-1-propanoic acid). Moreover, the thapsigargin-induced noradrenergic hyperresponsiveness, as well as that produced by subthreshold concentrations of the thromboxane A(2) mimetic, U 46619, were blocked by the Ca(2+) channel antagonist, verapamil. In conclusion, our results indicate that thapsigargin enhances the contractions produced by sympathetic nerve stimulation in human saphenous vein rings through the endothelial release of thromboxane A(2) that potentiates the vasoconstriction induced by the noradrenergic mediator with a verapamil-sensitive mechanism.

EDHF contributes to strain-related differences in pulmonary arterial relaxation in rats

Pulmonary arteries from the Madison (M) strain relax more in response to acetylcholine (ACh) than those from the Hilltop (H) strain of Sprague-Dawley rats. We hypothesized that differences in endothelial nitric oxide (NO) synthase (eNOS) expression and function, metabolism of ACh by cholinesterases, release of prostacyclin, or endothelium-derived hyperpolarizing factor(s) (EDHF) from the endothelium would explain the differences in the relaxation response to ACh in isolated pulmonary arteries. eNOS mRNA and protein levels as well as the NO-dependent relaxation responses to thapsigargin in phenylephrine (10(-6) M)-precontracted pulmonary arteries from the M and H strains were identical. The greater relaxation response to ACh in M compared with H rats was also observed with carbachol, a cholinesterase-resistant analog of ACh, a response that was not modified by pretreatment with meclofenamate (10(-5) M). N(omega)-nitro-L-arginine (10(-4) M) completely abolished carbachol-induced relaxation in H rat pulmonary arteries but not in M rat pulmonary arteries. Precontraction with KCl (20 mM) blunted the relaxation response to carbachol in M rat pulmonary arteries and eliminated differences between the M and H rat pulmonary arteries. NO-independent relaxation present in the M rat pulmonary arteries was significantly reduced by 17-octadecynoic acid (2 microM) and was completely abolished by charybdotoxin plus apamin (100 nM each). These findings suggest that EDHF, but not NO, contributes to the strain-related differences in pulmonary artery reactivity. Also, EDHF may be a metabolite of cytochrome P-450 that activates Ca(2+)-dependent K(+) channels.

Role of endothelium in thapsigargin-induced arterial responses in rat aorta

We assessed the role of endothelium in the arterial response to thapsigargin, the Ca(2+)-ATPase inhibitor of the endoplasmic reticulum, in rat isolated aortic rings. Thapsigargin induced an endothelium-dependent relaxation of phenylephrine-contracted aortic rings with an EC(50) of 2.6+/-0.4 nM and a 75% maximum relaxation, while it was less effective against 30 mM K(+)-induced contraction. Pretreatment of aortic rings with N(G)-nitro-L-arginine methyl ester (30 microM) or methylene blue (1 microM) reduced thapsigargin-induced relaxation by approximately 85%. Thapsigargin failed to relax the endothelium-denuded rings. L-Arginine (3 mM) partially, but significantly, antagonized the effect of 30 microM N(G)-nitro-L-arginine methyl ester. Pretreatment with indomethacin (3 microM), glibenclamide (1 microM) or iberiotoxin (100 nM) did not alter the thapsigargin-induced relaxation. In contrast, pretreatment with tetrapentylammonium ions (TPA(+), 1-3 microM) or with 300 microM Ba(2+) suppressed the relaxant response to thapsigargin. TPA(+) (3 microM) also attenuated acetylcholine-induced relaxation. Thapsigargin-induced endothelium-dependent relaxation was primarily dependent on the presence of extracellular Ca(2+). Interestingly, when the tissues were exposed to very low concentrations of thapsigargin (1-3 nM) the nitric oxide-dependent relaxation induced by acetylcholine or A23187 was markedly reduced. While thapsigargin (3 nM) did not influence the relaxation induced by endothelium-independent dilators, sodium nitroprusside and verapamil. These results indicate that thapsigargin produced complex vascular effects primarily by acting on the endothelial cells. Thapsigargin causes an endothelial nitric oxide-dependent relaxation; on the other hand, it inhibits nitric oxide-mediated relaxation at the similar concentrations. Activation of TPA(+)- and Ba(2+)-sensitive but not Ca(2+)-activated or ATP-sensitive K(+) channels may be also involved in thapsigargin-induced relaxation of rat isolated aortic rings.

Possible involvement of Ca2+ entry and its pharmacological characteristics responsible for endothelium-dependent, NO-mediated relaxation induced by thapsigargin in guinea-pig aorta

Thapsigargin, a specific inhibitor of Ca(2+)-pump Ca(2+)-ATPase in the sarcoplasmic/endoplasmic reticulum (SR/ER), produces an endothelium-dependent vascular relaxation. In the present study, pharmacological features of thapsigargin-induced endothelium-dependent relaxation were functionally characterized in the isolated guinea-pig aorta especially focusing on the Ca2+ mobilization mechanisms in endothelial cells. Thapsigargin-induced endothelium-dependent vascular relaxation was markedly suppressed by N(G)-nitro-L-arginine (L-NNA) and calmidazolium, suggesting that the vascular relaxation to thapsigargin is largely attributable to endothelium-derived nitric oxide (NO) produced as a result of the activation of Ca2+, calmodulin-dependent NO synthase (NOS). Removal of Ca2+ from the external solution abolished the endothelium-dependent relaxation of guinea-pig aorta in response to thapsigargin. Thapsigargin-induced endothelium-dependent relaxation was inhibited more strongly compared with the endothelium-independent relaxation to an NO donor, SIN-1 (3-(4-morpholinyl)-sydnonimine), when the artery preparation was preconstricted with a high concentration (80 mM) of KCl instead of agonistic stimulation. Endothelium-dependent relaxation induced by thapsigargin was not affected by diltiazem, a blocker of L-type voltage-gated Ca2+ channels. SK&F96365 (1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1 H-imidazole) and Ni2+, both of which block capacitative Ca(2+) entry, did not show any appreciable inhibitory effects on the endothelium-dependent relaxation to thapsigargin. These findings suggest that in guinea-pig aorta, endothelium-dependent NO-mediated relaxation induced by thapsigargin is preceded by the increase in the cytosolic free Ca2+ concentrations ([Ca2+]cyt) following the depletion of stored Ca2+ in thapsigargin-sensitive store sites in endothelial cells. Although the increase in [Ca2+]cyt responsible for the activation of endothelium NOS leading to thapsigargin-induced vascular relaxation may be ascribed to the capacitative Ca2+ entry from extracellular space, the Ca2+ entry mechanism stimulated with thapsigargin is deficient in sensitivity to SK&F96365 and Ni2+ in the endothelium of guinea-pig aorta.

Role of intracellular calcium for noradrenaline-induced depolarization in rat mesenteric small arteries

We have investigated the effect of intracellular calcium levels for membrane potential during noradrenaline application in isolated small arteries. Rat mesenteric small arteries were mounted for isometric tension measurement. Smooth muscle membrane potentials were measured by conventional intracellular electrodes, and intracellular calcium concentration was measured using Fura-2 fluorescence. Under control conditions, noradrenaline caused contraction and depolarization from -55.5 to -29.3 mV. In intact arteries, depleting intracellular calcium stores with thapsigargin caused smooth muscle hyperpolarization and inhibited contraction to noradrenaline. In de-endothelialized vessels, thapsigargin still depleted calcium stores, but did not affect either the depolarization or contraction caused by noradrenaline. In noradrenaline-activated vessels, inhibition of calcium influx by amlodipine caused tension and calcium levels to fall to near-baseline levels, but membrane potential returned by only 55%. Treatment with a combination of thapsigargin, D-600 and BAPTA-AM inhibited the tension and calcium responses to noradrenaline, but the membrane potential response was reduced by only 34%. Acute reduction of extracellular chloride concentration caused similar, small depolarization at rest and during noradrenaline exposure. It is concluded that an elevation of intracellular calcium concentration is not essential for noradrenaline depolarization, although part of the depolarization is associated with the raised intracellular calcium level.

Thapsigargin inhibits the response to acetylcholine and substance P but does not interfere with the responses to endothelium-independent agents

We investigated the influence of the Ca(2+)-ATPase inhibitor thapsigargin (TG) on the vasorelaxant response to different endothelium-dependent and endothelium-independent relaxing agents in an isolated thoracic aorta preparation of the rabbit, precontracted by norepinephrine (NE). Pretreatment with 100 microM L-arginine methyl ester (L-NAME) an inhibitor of nitric oxide (NO) synthesis, completely prevented acetylcholine (ACh)-induced relaxation; the inactive stereoisomer D-NAME did not modify the effect of ACh. The exposure of the preparations to 1 microM TG induced a slowly developing slight increase in the basal tension during 30-min contact. The same concentration of TG also slightly reduced the response to the subsequent administration of NE. The antagonist effect of TG on the ACh response was concentration dependent in the range between 0.1 and 10 microM. A 30-min pretreatment with 1 microM TG appeared to be sufficient to induce a consistent antagonism of the ACh (0.01-10 microM) concentration-relaxant effect curve, since an increase to 60 min did not produce a further significant increment in the degree of the antagonist effect. The concentration-dependent relaxation induced by substance P (SP 0.1-3 nM) was also significantly antagonized by 1 microM TG. The effect of the calcium ionophore A23187 (0.01-1 microM) was reduced by the Ca(2+)-ATPase inhibitor only at the higher concentrations tested (0.3-1 microM). However, a 30-min contact time with 1 microM TG was completely ineffective in antagonizing the concentration-relaxant response curves to the two nitrovasodilators sodium nitroprusside (SNP 0.1-100 microM) and nitroglycerin (NTG 1-300 nM) and to the cyclic GMP analogue 8-Bromo-cyclic GMP (3-100 microM). The effects of the beta-adrenoceptor agonist isoprenaline (ISO 0.1-10 microM) and of the direct adenylate cyclase activator forskolin (FK 0.01-10 microM) were also completely unaffected by 1 microM TG. These results demonstrate that TG affects the response to agents that induce an endothelium-dependent relaxation through receptor-dependent calcium mobilization. However, they do not support the hypothesis that sarcoplasmic pump activity is essential for the development of a vasorelaxant response to endothelium-independent agents.

Relaxation of arterial smooth muscle by calcium sparks

Local increases in intracellular calcium ion concentration ([Ca2+]i) resulting from activation of the ryanodine-sensitive calcium-release channel in the sarcoplasmic reticulum (SR) of smooth muscle cause arterial dilation. Ryanodine-sensitive, spontaneous local increases in [Ca2+]i (Ca2+ sparks) from the SR were observed just under the surface membrane of single smooth muscle cells from myogenic cerebral arteries. Ryanodine and thapsigargin inhibited Ca2+ sparks and Ca(2+)-dependent potassium (KCa) currents, suggesting that Ca2+ sparks activate KCa channels. Furthermore, KCa channels activated by Ca2+ sparks appeared to hyperpolarize and dilate pressurized myogenic arteries because ryanodine and thapsigargin depolarized and constricted these arteries to an extent similar to that produced by blockers of KCa channels. Ca2+ sparks indirectly cause vasodilation through activation of KCa channels, but have little direct effect on spatially averaged [Ca2+]i, which regulates contraction.

Thapsigargin- and cyclopiazonic acid-induced endothelium-dependent hyperpolarization in rat mesenteric artery

1. The present study was designed to determine whether putative, selective inhibitors of the Ca(2+)-pump ATPase of endoplasmic reticulum, thapsigargin (TSG) and cyclopiazonic acid (CPA), induce endothelium-dependent hyperpolarization in the rat isolated mesenteric artery. The membrane potentials of smooth muscle cells of main superior mesenteric arteries were measured by the microelectrode technique. 2. In tissues with endothelium, TSG (10(-8)-10(-5) M) caused sustained hyperpolarization in a concentration-dependent manner. In tissues without endothelium, TSG did not cause any change in membrane potential. CPA (10(-5) M) also hyperpolarized the smooth muscle membrane, an effect that was endothelium-dependent and long-lasting. 3. The hyperpolarizing responses to these agents were not affected by indomethacin or NG-nitro-L-arginine (L-NOARG). 4. In Ca(2+)-free medium, neither TSG nor CPA elicited hyperpolarization, in contrast to acetylcholine which generated a transient hyperpolarizing response. 5. In rings of mesenteric artery precontracted with phenylephrine, TSG and CPA produced endothelium-dependent relaxations. L-NOARG significantly inhibited the relaxations to these agents, but about 40-60% of the total relaxation was resistant to L-NOARG. The L-NOARG-resistant relaxations were abolished by potassium depolarization. 6. These results indicate that TSG and CPA can cause endothelium-dependent hyperpolarization in rat mesenteric artery possibly by releasing endothelium-derived hyperpolarizing factor and that membrane hyperpolarization can contribute to the endothelium-dependent relaxations to these agents. The mechanism of hyperpolarization may be related to increased Ca2+ influx into endothelial cells triggered by depletion of intracellular Ca2+ stores due to inhibition of endoplasmic reticulum Ca(2+)-pump ATPase activity.

Inhibition of calcium release from the sarcoplasmic reticulum of rabbit aorta by hydralazine

1. The mechanism of hydralazine-induced vasorelaxation was investigated in rabbit isolated aorta, by determining its ability to interfere with force development under a variety of conditions. 2. Hydralazine relaxed phenylephrine-contracted aorta with half maximal relaxation at 17 microM and maximal relaxation above 100 microM. At 200 microM, hydralazine had little effect on contractions induced by 25 mM or 50 mM K+. 3. Hydralazine was equally effective at inhibiting contractile responses to phenylephrine in the absence or presence of extracellular Ca2+. Responses to phenylephrine in Ca(2+)-free solution were blocked to the same degree whether hydralazine was applied during filling of the sarcoplasmic reticulum (SR) Ca2+ stores or after filling was complete. Caffeine-induced contractions were less sensitive to block by hydralazine. 4. Thapsigargin, cyclopiazonic acid, ryanodine, nifedipine and diltiazem all failed to block the inhibitory effect of hydralazine on tonic contractions to phenylephrine in the presence of extracellular Ca2+. However, when cyclopiazonic acid was applied either with diltiazem or ryanodine, substantial inhibition of the hydralazine response was observed. 5. We propose that tonic contractions to phenylephrine are largely maintained by Ca2+ cycling through the SR, with Ca2+ entering the smooth muscle cell being sequestered by the SR eventually to leak out through IP3-activated channels close to the contractile proteins. Sequestration of Ca2+ would employ two pathways, one sensitive to inhibitors of the SR Ca(2+)-ATPase and the other to Ca antagonists. We further suggest that, in the presence of extracellular Ca2+ and phenylephrine, the leakage of Ca2+through IP3-activated channels is significantly reduced only if both routes for SR Ca2+ accumulation are blocked or the Ca2+-ATPase is blocked while the SR is made leaky with ryanodine.6. We conclude that the main action of hydralazine is to block the IP3-dependent release of Ca2+ from the sarcoplasmic reticulum. Thus conditions that diminish the contribution of IP3-induced Ca2+ release to tension can inhibit the hydralazine-induced vasorelaxation.